1. Field of the Invention
The present invention relates to cancellation of an input offset voltage applied to a differential voltage amplifier circuit, and particularly to a differential voltage amplifier circuit capable of obtaining high accuracy detection of a differential voltage.
2. Description of the Related Art
There has recently been an increasingly demand for a high accuracy current detecting means. For example, various portable electronic equipments such as a notebook personal computer, etc. need to bring the measurement of a remaining capacity of a second battery to high precision. For that purpose, the high accuracy current detecting means is required.
A technique disclosed in Japanese Unexamined Patent Publication No. 7(1995)-191110 will be explained using FIG. 7 as a related art. A circuit according to the related art shown in FIG. 7 is equipped with a sense resistor 102 which detects charge and discharge currents from a secondary battery 101, a differential voltage amplifier circuit 104 which amplifies a terminal voltage applied across the sense resistor 102, and a switch 103 which short-circuits two input terminals of the differential voltage amplifier circuit 104 to temporarily bring an input difference voltage to zero. A microcontroller 105 includes an A/D converter 107, a current detection circuit 108, a remaining capacity arithmetic circuit 109 and a switch control circuit 110.
The output of the differential voltage amplifier circuit 104 at the time that the switch 103 short-circuits the two input terminals of the differential voltage amplifier circuit 104 to bring the input difference voltage to zero, is converted into a digital value by the A/D converter 107, which in turn is determined as an offset voltage value in advance and stored. Then, a value obtained by adding or subtracting the previously-determined offset voltage value to and from an output voltage value of the differential voltage amplifier circuit 104 at the time of amplification of the terminal voltage of the sense resistor 102, is used as a value corresponding to the charge or discharge current. The remaining capacity arithmetic circuit 109 performs an arithmetical operation on the remaining capacity. It is thus possible to reduce an error in the result of the arithmetical operation on the remaining capacity of the secondary battery 101 due to the offset voltage.
In the method for detecting the input offset voltage value of the differential voltage amplifier circuit and adding and subtracting it to and from the output voltage value to thereby perform correction on the result of detection as described in the related art, the input offset voltage value is contained in the input voltage value of the differential voltage amplifier circuit. As a result, the originally provided input voltage allowable range of the differential voltage amplifier circuit is shifted by the input offset voltage. Thus, a range obtained by subtracting a range occupied by the input offset voltage therefrom results in the input voltage allowable range of the differential voltage amplifier circuit in the related art. Therefore, the measurement range is limited since the input voltage allowable range used for measurement is narrowed, thus causing a problem such as a reduction in accuracy of measurement.
When the input offset voltage changes due to temperatures, an error occurs between an output voltage value calculated for correction and an actual differential voltage, depending on measured temperatures. Since the remaining capacity of the battery is calculated on the basis of the integrated output voltage value, the measured error is also integrated and hence the error between the calculated remaining capacity of the battery and the value of an actual remaining capacity of the battery becomes large to such an innegligible extent, thus causing a problem. In order to avoid such a problem, allowance is made for the remaining capacity of the battery and there is no other choice but to issue a warning against a shortage of remaining capacity. Thus, a problem arises in that the capacity of the battery cannot be used to the full. When the input offset voltage value is measured for each differential voltage measurement, an increase in burden on an MPU, an increase in current consumption, etc. occur, thus causing a problem.
Since the input offset voltage value is being contained in the input differential voltage of the differential voltage amplifier circuit, it is necessary to perform correction calculation for eliminating the influence of the offset voltage value from the input differential voltage for each differential voltage measurement. Therefore, the MPU needs to perform arithmetic processing every correction calculations. Hence its control is cumbersome and current consumption also becomes large, thus causing a problem.
Since the input offset voltage is stored in the MPU, it is necessary to perform a test to calculate the input offset voltage and store it in the MPU under a high-precision measurement environment. Therefore, a problem that a testing device becomes large in scale, and a problem that testing becomes expensive arise, thus causing problems.